Many wireless channel digital frequency and phase modulation systems such as frequency shift keying (FSK), Gaussian frequency shift keying (GFSK), and minimum phase shift keying (MSK) are sensitive to carrier frequency offset (CFO) caused by transceiver oscillator instability and/or Doppler shift. This is especially true when data is transmitted in a burst mode. One possible solution applies an auto-frequency calibration (AFC) block in the receiver to automatically estimate and compensate for such frequency offset. However, compared with the multiplicity of conventional designs for FSK/GFSK/MSK transceivers, CFO estimation and compensation circuits for such systems are rare.
Conventional CFO estimation and/or compensation schemes have many drawbacks. An early conventional scheme utilized a set of analog AFC tracking algorithms and can be recognized as the basis of the modern digital AFC. Some conventional digital schemes utilized a set of digital closed-loop decision-aided AFC tracking algorithms for GFSK systems. However, both of these algorithms require reconstruction of transmitted data symbols and submission of these data symbols to the CFO estimator as reference information. Therefore, the trackable AFC range of these conventional schemes is limited so as to not exceed the maximum frequency divination and, thus, accurate sample timing recovery is required. Another typical scheme utilizes an open-loop AFC tracking algorithm which directly estimates the DC offset of the discriminator output. However, application of this AFC algorithm is limited to frequency modulation systems with discriminator demodulators. A direct CFO estimator based on received signals and remodulated transmitted symbols has also been proposed, but the channel response and the training sequence must be known in advance. Further, some conventional AFC algorithms are based on Fast Fourier Transforms (FFT) and Maximum Likelihood which have disadvantageous high computational requirements.
Additionally, many of the existing AFC algorithms assume that the received signal has a constant envelope. However, this assumption is not always true, especially when the Inter-Channel Interference (ICI) and Automatic Gain Control (AGC) uncertainties are taken into account. A further normalization method for AFC in GFSK systems normalizes the estimated CFO to the maximum deviation, ignoring gains along the receiving path.
Thus, what is needed is an easy to implement automatic frequency calibration scheme which does not require timing recovery and/or source data recovery while also taking into account the ICI and AGC uncertainties. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.